ULTRASOUND OF THE ADULT KIDNEY - Normal

For Renal Function Tests Explained

A comprehensive examination of the renal tracts should always include

assessment of the urinary bladder and, in males,the prostate.
 

                  SCAN PLANE                                                                                                        IMAGE                                                                                  

Coronal scan plane for the Right Kidney	Longitudinal: Normal Kidney

Scan plane transverse kidney	Transverse normal image

ANATOMICAL VARIANTS

In the 1st trimester, the developing kidneys ascend in the foetal abdomen. If the progress is hampered, this can result in:

• An ectopic kidney if it fails to reach the normal position.
• Crossed fused ectopia (both on one side)
• Or a horseshoe kidney if the lower poles fuse.

An interruption to the vascular supply to the developing kidney will result in an atrophic, poorly differentiated kidney.

  Common anatomical variants

• Atrophic small kidney
• Horseshoe kidney
• Ectopic kidney
• Duplex kidney
• Cross fused ectopia
• Unilateral renal agenesis
  

For a link to renal embryology

For another link to renal embryology

HORSESHOE KIDNEY

• Occurs when there is fusion of the metanephros as they are pushed together during their ascent from the sacral region.
• Almost always involves fusion of the lower poles.
• There is an increase incidence of infection, calculi and tumors in horseshoe kidneys.

Horseshoe kidney: A transverse view across the midline showing the isthmus across the aorta.	Horseshoe kidney: Longitudinal view of the horseshoe isthmus.

A sagittal view of the isthmus of a horseshoe kidney.

ECTOPIC KIDNEY

Also the result of abnormal or interrupted ascent during embryology.

• The most common ectopic site is in the pelvis. The kidney will lie obliquely in the ipsilateral iliac fossa.
• Less commonly, a kidney may ascend to the other side with 2 kidneys on one side of the abdomen. This is called crossed-ectopia. This may result in a single large fused kidney as shown below (crossed-fused-ectopia)

Cross fused ectopic kidney. The left kidney is fused to the lower pole of the right kidney.

To identify the cause of:

• Flank pain
• Haematuria (frank or microscopic)
• Follow-up of previously identified pathology
• Classification of a mass (Solid V's cystic)
• Post surgical complications
• Guidance of aspiration, biopsy or intervention
• Post injury

LIMITATIONS

• The mid to distal ureter is generally obscured by bowel gas.
• Small lesions at the upper pole of the kidney may be difficult to see due to refractive edge shadowing. This can be overcome with thorough scanning technique.

EQUIPMENT SELECTION

Highest frequency curved linear array probe possible. Start with 7MHz and work down to 2 or 3 for larger patients. Assess the depth of penetration required and adapt. Paediatric and thin patients should be scanned with a 7MHz. Good colour / power / Doppler capabilities when assessing vessels or vascularity of a structure.

Begin with the patient supine. Each kidney may also need to be examined in the decubitus position. Raise the ipsilateral arm above the patient's head.

TECHNIQUE

A comprehensive examination of the renal tracts should always include assessment of the urinary bladder and, in males,the prostate.

Scan longitudinally right subcostally. Visualise the kidney inferior to the right lobe of the liver (RT), or spleen (LT). Place the probe between iliac crest and the lower costal margin to examine in the coronal plane. Ensure the kidney is thoroughly examined from edge to edge. Rotate into transverse. Scan from beyond the superior margin to inferior. Document the normal anatomy and any pathology found, including measurements and vascularity if indicated.

WHAT TO CHECK

• Kidney size (should not be >1cm difference between sides)
• Cortical thickness(not <10mm)
• Cortico-medullary differentiation
• Cortex at least as hypoechoic as the liver
• Pyramids slightly hypoechoic relative to the cortex
• No hydronephrosis
• Renal scarring(beware mistaking prominant lobulations as scars)

• Calculus
• Renal cyst
  • Cortical
  • Parapelvic
• Angiomyolipma
• Renal cell carcinoma
• Transitional cell carcinoma
• Hydronephrosis
• Medullary sponge kidney
• Polycystic kidney disease

A renal series should include the following minimum images;

• Both kidneys with length measurements
• Right kidney long with liver for comparison
• Both kidneys longitudinal medial and lateral
• Both kidneys transverse
  • sup
  • mid
  • inf
• Left kidney long with spleen for comparison
• Document the normal anatomy. Any pathology found in 2 planes, including measurements and any vascularity.